Butterfly valves used for automatic control reduce pressure, particularly at low flow i.e. low disc openings. Such pressure reduction can only be accomplished by acelleration and subsequent decelleration of the passing medium, where the maximum velocity is a square root function of the pressure drop. Unfortunately, such high velocities along the leading edges of butterfly valves produce undesireable side effects. Such side effects are cavitation with liquid media, aerodynamic throttling noise with gases, and a high dynamic torque with either medium. Such dynamic torque, increasing rapidly towards the fully open position and reaching a peak near 70.degree. disc opening (see curve A in FIG. 4), greatly interferes with the stable valve operation particularly when pneumatic actuators are employed, and is a function of the suction effect (much like the "lift" of an airplane wing) produced by high velocity on the upper surface of that portion of the disc pointing in the downstream direction.
Past butterfly discs of improved designs have tried to overcome this dynamic torque problem, notably among them a disc whose terminating downstream periphery has the shape of a fishtail. Another design employs semi-circular cavities on opposing sides of the circular disc. While those designs show improvements in reduction of dynamic torque, neither of them meets the additional objections of my invention, which are:
A. reduce cavitation; PA1 B. provide tight shut-off; PA1 C. create a better control characteristic; PA1 D. have a low aerodynamic noise level. My invention produces substantially less dynamic torque by creating local vortexes around the outer periphery of the disc produced by vertical ribs which break up the uniform velocity profile on the upper disc surface, similar to "spoilers" on top of airplane wings. Furthermore, rather than creating a single semi-circular jet on each half of a present state of the art disc, which attaches to the pipe wall and can create cavitation and noise, my invention in contrast provides for small individual streams which discharge into a relatively large volume (in comparison to the cross-section of the jet) thereby losing nearly all of its velocity head, i.e. having very little "pressure recovery". Low pressure recovery means lower initial jet velocity, therefore lower dynamic torque and less chance for cavitation. For gaseous media, this system produces considerably less aerodynamic noise, since the sound pressure level of a jet increases to the eighth power of its velocity. This means in practice, that if the velocity (due to change in pressure recovery from 80% to 20% for example) can be reduced to half its original value (for the same pressure drop), then the sound power is reduced 256 times-
My invention also provides means to combine tight shut-off with the other aforementioned advantages, in that its configuration (contrary to other "low torque" designs) allows an angular attachment, usually between 15.degree. and 20.degree. to the vertical valve axis, of the outer disc periphery to provide metallic contact and thereby greatly reduced valve leakage. The angle of contact is chosen depending on the type of material employed, that is the tangent of the angle has to exceed the coefficient of static friction to avoid self-locking.
A further object of my invention, as mentioned before, is the ability to create a better relationship between flow rate and valve stroke i.e. disc rotation. The most desired characteristic of this type is called "equal percentage", i.e. where the flow rate increases in equal percentage values from the previous rate for each degree of disc rotation. Such characteristic requires relatively little flow rate at the beginning of the stroke but which rapidly increases near the fully open position. My invention compliments such desire by providing means of chosing the amount of flow area desired to match the characteristic at least up to about 50% of disc rotation by varying the flow passages dimensions between the individual ribs. Yet, the ribs do not interfere materially with the maximum flow rate, since they do not extend (at 100% opening) materially beyond the profile of the disc hub.
These and other advantages and objections will be more apparent when viewed in light of the following detailed description and drawings.